The present invention relates generally to the problem of aluminum alloy workpieces absorbing hydrogen when undergoing heat treatment in furnaces containing ambient moisture-laden atmospheres, and particularly to an etch cycle in making aluminum components using a transition metal chloride salt in the final step of the etch cycle, the salt being added to either an alkaline or acidic etch bath.
When an aluminum alloy object is heated in the presence of moist air, a protective oxide layer on the aluminum object is invariably disrupted to expose nascent aluminum. Aluminum oxidation in the presence of water, while in a heated furnace, generates atomic hydrogen, which readily diffuses into the aluminum object, and is the only gas that has appreciable solubility in aluminum. Still, atomic hydrogen has limited solubility in metal and has the propensity to precipitate in the metal as insoluble molecular hydrogen (H2) at heterogeneities or defects, especially in highly worked regions within the metal object. As increasing hydrogen is precipitated within pores in the metal, additional hydrogen can be absorbed and accommodated within the metal matrix. Bulk porosity in an aluminum workpiece can compromise structural integrity and the mechanical performance of the final aluminum part.
For several decades, ammonium fluoborate (NH4BF4) protective atmospheres have been used in the industry to prevent substantial absorption of hydrogen by aluminum alloy workpieces during high temperature furnace treatments. Ammonium fluoborate decomposes during such treatments at temperatures above 482xc2x0 F. to form a blanket atmosphere that fills the entire internal volume of a furnace. Ammonium fluoborate also produces an array of compounds in the furnace which can eliminate high temperature oxidation reactions by either reacting with ambient water or by forming a protective fluorinated layer on the aluminum alloy workpiece.
There are drawbacks to the use of ammonium fluoborate atmospheres, however. Ammonium fluoborate species can stain and pit surfaces of some aluminum alloys. The ammonium fluoborate decomposition products contain toxic, corrosive and particulate species. The ammonium fluoborate emissions corrode furnace structures and baghouses for filtering particulate emissions. Disposal of the collected particulates is costly. Concerns relating to the emissions have prompted research to identify alternative chemistries that are more environmentally friendly and safer for in-plant use.
During certain typical processing sequences in forging aluminum components, the surfaces of the components are frequently subjected to an etch cycle between fabrication stages to enable better surface inspection and possible hand repair of surface defects. Such etch cycles can consist of a caustic etch and a nitric acid desmut, each followed by one or more water rinses. The caustic etch removes lubricant and excess oxide that accumulates during the forging processes. The nitric acid desmut removes smut, a dark residual film of alloying constituents that have low solubility in a caustic bath, especially copper, magnesium and silicon containing phases. Often, the caustic etch is preceded by a nitric etch, which is believed to weaken bonding of residual lubricant on the component surfaces.
The present invention involves a simple modification of an existing etching operation to provide a protective chemistry on aluminum workpieces prior to being heated in moisture-laden atmospheres and worked at high temperatures. The protective chemistry of the invention can be applied during a final nitric acid desmut step, in which case the composition of the nitric acid solution is modified by addition of 5 to 25 wt. % transition metal chloride salt and elimination of the final rinse. During subsequent heat treatments, the deposited chemistry on the etched aluminum surface consumes any atomic hydrogen generated by high temperature surface oxidation reactions or outgassed from the aluminum bulk. Experimentation has shown the efficacy of adding 10 parts ferric chloride to 90 parts of a 39 wt. % nitric acid solution, the latter being a typical desmutting composition. As explained in detail below, the hydrogen contents of aluminum samples treated with such a composition remained low during a subsequent heat treatment in a moisture-laden atmosphere.